Hydrogen has been suggested as an ecologically clean energy carrier because it does not produce air pollution or the greenhouse gases carbon dioxide, carbon monoxide or methane. Hydrogen has almost three times the energy content of gasoline based on weight, but only about a quarter based on volume (Satyapal et al., Catal. Today 120:246-256, 2007). Lower-cost, lighter-weight and higher-density hydrogen storage is one of the key requirements for hydrogen energy use. The US Department of Energy (DOE) has set technology targets for hydrogen storage for 2010 and 2015. It is desired by 2010 to develop hydrogen storage systems achieving a gravimetric density of 2 kWh/kg (6 wt. %), a volumetric density of 1.5 kWh/L, and a cost of $4/kWh, and by 2015, corresponding quantities of 3 kWh/kg (9 wt. %), 2.7 kWh/L, and $2/kWh. Four methods are being considered by the DOE as candidates for hydrogen storage: compression and storage in cryogenic tanks, adsorption by metal hydrides, adsorption on high surface area materials, and chemical hydrogen storage (including off-board regeneration) (Hydrogen storage roadmap.
High pressure storage needs high strength containers and has a limited volume capacity. A conventional steel hydrogen cylinder can hold only 1% by weight hydrogen and the boil-off of liquefied hydrogen requires venting, reduces driving range, and produces safety problems. Hydrogen liquefaction is also energy intensive at an expense of 30% of the heating value of hydrogen.
Metal hydrides are difficult to apply because they are too thermodynamically stable. This has two consequences. First, the hydrides have to be heated to an inconveniently high temperature to release hydrogen. Second, the heat of absorption is so high that a large amount of heat must be removed during the refueling process. Adsorption of hydrogen onto high surface area materials, such as carbon nanotubes, has been studied but also has barriers, including, for example, reproducibility of the material synthesis and hydrogen storage performance (Anson et al., Nanotechnology 15:1503, 2004). Compared with the above methods, chemical hydrogen storage provides high gravimetric and volumetric hydrogen densities. Additionally, chemical hydrogen storage has an advantage that hydrogen storage and transportation use conventional petrochemical substances (Biniwale et al., Int. J. Hydrogen Energy 33:360-365, 2008).
U.S. Pat. No. 3,479,165 discloses a hydrogen storage system using MgH2 (magnesium hydride) that is decomposed at high temperatures and pressures. This patent was issued in 1969 and already recognized that magnesium hydride allegedly met the key criteria “easily reversible, fast reaction rates, side reactions eliminated to maintain purity, high weight percent hydrogen and low compound weight.” Yet the DOE (Department of Energy) has focused most attention on one criterion, the energy density on a weight basis for hydrogen that can be released. Other key criteria of reversibility, speed of reaction rates and side reactions/purity appear not to have been considered. However, even in the 1960's, U.S. Pat. No. 3,479,165 further states: “it is noted that organic compounds containing hydrogen, although light in weight, are not free of side reactions which tend to severely limit the hydrogen purity. Thus, organic compounds also, are not suitable for hydrogen storage and transportation.”
Organic chemical hydrides employ hydrogenation-dehydrogenation of cyclic hydrocarbons or heteroaromatic compounds as a means to store and transport hydrogen. Aromatic compounds, such as benzene, toluene, and naphthalene can be hydrogenated by using appropriate metal catalysts under relatively mild conditions, e.g. about 100° C. and 2 MPa. However, the dehydrogenation of cyclic hydrocarbons is endothermic and the reaction is favored only at high temperatures as well as having problems with coking on catalyst surfaces requiring catalyst regeneration every 1-2 hours. Catalytic dehydrogenation under “liquid-film state” conditions has been reported (Meng et al., Int. J. Hydrogen Energy 22:361-367, 1997; Hodoshima et al., Int. J. Hydrogen Energy 28; 197-204, 2003; Hodoshima et al., Appl. Catal. A: Gen. 292:90-96, 2005; and Hodoshima et al., Appl. Catal. A: Gen. 283:235-242, 2005), where the reactant is supplied as a liquid so that the surface of the catalyst is wetted with a thin film. Equilibrium limits were surpassed because of evaporation of the dehydrogenated reactants. Another method uses “wet-dry multiphase conditions” to take advantage of multiple phases to get over thermodynamic equilibrium limitations (Kariya et al., Appl. Catal. A: Gen. 247:247-259, 2003; and Kariya et al., Appl. Catal. A: Gen 233:91-102, 2002). However, both processes still require relatively high temperatures for vaporization of the volatile components of the process. An important need is also an effective separation of hydrogen from the mixtures to get a pure hydrogen product and to reuse the hydrogen carrier materials.
Heteroatom aromatic rings for H2 storage were proposed because the addition of electron-donating groups favors H2 release both thermodynamically and kinetically at moderate temperatures. In the case of indoline, dehydrogenation is possible at modest temperature (110° C.) (Moores et al., New J. Chem. 30:1675-1678, 2006). Benzimidazolines, including N,N′-dimethyldihydrobenzimidazole, 1,3-dimethyl-2-phenylbenzimidazoline, and 1,3-dimethylbenzimidazoline, were studied with different palladium catalysts, releasing H2 even at 80° C. (Schwarz et al., Chem. Commun. 5919-5921, 2005).
However, hydrogen density is an important factor in hydrogen storage, according to the DOE. Therefore, a lower weight of the organic framework is desired while maintaining favorable thermochemical and kinetic parameters. Smaller molecules, such as 4-aminopiperidine and piperidine-4-carboxamide are proposed compounds for reversible hydrogen storage (Cui et al., New J. Chem. 32:1027-1037, 2008). Dehydrogenation and hydrogenation of 4-aminopiperidine and piperidine-4-carboxamide occur at low temperatures without by-products, such as C—N cleavage and hydrogenolysis products. Dehydrogenation may be favored in five-membered rings over six member rings and by the incorporation of N heteroatoms into the rings, either as ring atoms or as ring substituents, particularly in 1, 3 positions (Clot et al. Chem. Commun. 2231-2233, 2007). Heteroaromatic ligands have been used for reversible hydrogenation/dehydrogenation, specifically N-ethyl carbazole hydrogenated with 72 atm and a Pd catalyst at 160° C. and dehydrogenated with Ru at 50-197° C. (U.S. Pat. No. 7,351,395, the disclosure of which is incorporated by reference herein). A thiol-based system is proposed in U.S. Pat. No. 7,186,396, the disclosure of which is incorporated by reference herein.
Therefore, the present disclosure provides a group of organic compounds that, under the specified reaction conditions, can satisfy the key criteria of easily reversible, fast reaction rates with minimal side reactions, in addition to the DOE criteria of high weight percent hydrogen and low compound weight.